It is well known that for high speed passenger ships a ride control system using trim-tabs or/and fins or interceptors is very effective to reduce ship motions in waves and to improve ride comfort of passengers on it. In the first part of the study, measurements of hydrodynamic forces created by trim-tabs and interceptors are carried out to know the hydrodynamic characteristics of these two devices. In such a ride-control system, a conventional PID control is usually used. In the second part, ship motions of the scale model of a fast car-ferry with these control devices under PD control in head waves are measured to assess the performance of the control system. The results show these devices are not always effective for all frequency range of waves, and show lower performance in high and low frequency regions. In order to improve the performance of the control system in the high frequency region, a new hybrid control system is developed and proposed in the third part. In the system, the wave exciting pitch moment is estimated using a linear equation of motion using hydrodynamic derivatives predicted by a strip method, and is compared with the maximum moment created by these control devices. When the estimated exciting moment is smaller than the moment created by these control devices, they are controlled to cancel the wave exciting moment, otherwise a PID control is adopted. These two control methods are automatically selected in the system. In order to confirm the effectiveness of the proposed system, a time-domain simulation using a non-linear coupling equation for heave and pitch is carried out in regular and irregular head waves. The results show the system can reduce pitch motions of a fast ship in head waves significantly.
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